The variations of blood pressure occurring during various physiological states of a patient is of great interest in modern medical diagnostic procedures. The traditional method of characterizing blood pressure is a determination of the systolic and diastolic pressure values. Another measurement variable, the mean arterial pressure (MAP), has also been determined to be useful as an indication of blood pressure. The mean arterial blood pressure is defined as the time average of the instantaneous blood pressure or as a weighted average of the systolic and diastolic pressures. In particular, if blood pressure is plotted relative to time, the MAP is a level chosen so that the area between the systolic section of the curve and the MAP level equals the area between the MAP level and the diastolic section of the curve. The MAP level can be roughly estimated from the systolic and diastolic values according to the following formula: EQU MAP=Diastolic+1/3(Systolic-Diastolic)
The value determined by this equation may be inaccurate in shock cases, in an operating room environment, or where certain diseases are involved due to changes in the blood pulse waveform.
There are presently several methods of measuring the various values of arterial blood pressure which are in common use. The most accurate method is direct measurement of arterial pressure by using an arterial cannula. However, invasive techniques are often inconvenient and may give rise to considerable patient discomfort.
Accordingly, several noninvasive techniques have been developed. One of the earliest techniques which is in common use involves occluding the blood vessels in a patient's limb by means of inflatable cuff which encircles the limb. The pressure (typically, air) in the cuff is then slowly decreased. When the decreasing pressure equals the arterial systolic pressure, characteristic sounds commonly known as Korotkoff sounds can be heard by auditory monitoring of the blood flow. When the decreasing pressure in the cuff reaches the arterial diastolic pressure, the Korotkoff sounds also change in a characteristic manner. These phenomena can be easily used to measure the systolic and diastolic blood pressure by observing the cuff pressure by means of conventional mercury or aneroid spnygmomanometer while manually listening to the blood flow in the arteries. The technique has also been automated by detecting the Korotkoff sounds using microphones or ultrasound transducers in the inflatable cuff. One problem with this method is that it cannot be used to directly measure the mean arterial pressure which must be estimated from the systolic and diastolic values using the formula referred to above. This formula may be inaccurate due to a variety of factors including disease or shock.
A more recently discovered technique is the oscillometric method of detecting and quantifying blood pressure values. This technique utilizes a blood vessel-occluding air cuff as in the Korotkoff technique, but senses blood pressure values by a different means. Specifically, as the air pressure in the inflatable air cuff is decreased below the systolic blood pressure, small pressure oscillations can be observed above the baseline cuff pressure. These small pressure oscillations are reflected in the air pressure of the surrounding cuff as result of expansion and contraction of the arteries produced by the pulsatile blood flow. The pressure oscillations increase in amplitude and reach a maximum as the cuff pressure becomes equal to the mean arterial blood pressure. The oscillations then decrease in amplitude until they entirely disappear below a threshold value of the applied cuff-pressure. The mean arterial pressure is then easily measured by detecting the air cuff pressure at which the maximum amplitude of the pressure oscillations in the air cuff occurs. This measurement technique is easily automated and is especially useful in blood pressure and measuring devices that are controlled by microprocessors.
However, one problem with prior art blood pressure measuring devices using the oscillometric method is that although the mean arterial pressure can easily be measured, no simple, accurate method for measuring either systolic or diastolic pressures has been developed.
Consequently, most prior art devices rely on an extrapolation of the systolic and diastolic pressures from the measured mean arterial pressure. For example, it has been observed that the systolic and diastolic pressures occur at points where the pressure oscillations in the air cuff reach a magnitude which is approximately one half the magnitude of the oscillations at the mean arterial pressure. This method provides an easy way of calculating the systolic and diastolic pressures from the mean arterial pressure. However, it is subject to several additional problems. First, artifacts introduced by patient movement or outside interference may produce erroneous results if they occur at cuff pressure measurements in the vicinity of the diastolic or systolic pressures. Secondly, the one-half magnitude relation of the oscillation amplitudes at mean pressure and systolic/diastolic pressures is not exactly correct. Therefore the systolic and diastolic pressures calculated by this technique are only approximations as to the true systolic and diastolic pressures.
Accordingly, it is an object of the invention to provide a more accurate method for determining systolic and diastolic blood pressure values in an oscillometric-mode blood-pressure measuing system.
It is another object of the invention to obtain accurate systolic and diastolic blood pressure readings in the presence of noise and other external disturbances and in the case of shock, operating room environments and disease situations.
It is a further object of the invention to obtain increased artifact rejection in obtaining systolic and diastolic blood pressure readings.